1. Field of the Invention
The present invention is directed to a face shooter ink jet printing head and to a method for the manufacture thereof.
2. Description of the Prior Art
Ink jet printer heads are employed in small, fast printers, for example, postage meter machines for franking postal items.
Ink jet printing heads can be constructed according to the edge shooter principle or according to the face shooter principle (First Annual Ink Jet Printing Workshop, Mar. 26-27, 1992, Royal Sonesta Hotel, Cambridge, Mass.). Heretofore, efforts were directed to minimizing the dimensions of the chambers in order to increase the nozzle density. These measures, however, are only meaningful given ink jet modules having a few nozzles in a row and are not useful given a high number of nozzles.
German OS 32 48 087 discloses a face shooter liquid jet head, one version of which has nozzle groups lying in a nozzle line such that neighboring lead-zirconate-titanate elements (PZT elements) are separately supplied with fluid. A manifold-like branching of the ink channels leads from the delivery channel to the elements at each side. The longitudinal axes of the ink chambers lie in the direction of the ink jet emission from the face shooter nozzles. The width of the chamber size under the PZT elements is limited by this arrangement and a high nozzle density is not achieved.
Ink jet printing heads according to the face-shooting principle that were developed later, as disclosed, among others, in U.S. Pat. No. 4,730,197, U.S. Pat. No. 4,703,333, U.S. Pat. No. 4,695,584, U.S. Pat. No. 4,635,079, U.S. Pat. No. 4,641,153 and U.S. Pat. No. 4,680,595 are likewise composed of ink chambers that are orthogonally arranged relative to the longitudinal axis of the ink chambers to the left and right of a line of nozzle exit openings. The ink chambers all have their longitudinal axis lying in one plane. In this arrangement, as in the aforementioned arrangement, the achievable density in the arrangement of the nozzles is defined by the width of the chambers and by the thickness of the partition lying between two chambers. This partition cannot fall below a specific minimum thickness because otherwise cross-talk occurs. The arrangement which is undertaken at both sides of and symmetrically relative to the nozzle line only achieves a doubling of the nozzle density. Geometrical resolutions of 64 dpi can be currently achieved with such arrangements. This resolution, however, is not adequate for printing graphic symbols as required, for example, by label printers or postage meter machines.
In particular, U.S. Pat. No. 4,680,595 discloses a manufacturing method for a face shooter ink jet printing head having a nozzle line between two groups of ink chambers which has double the nozzle density. A chamber plate that carries the chambers in a symmetrical arrangement relative to the nozzle line is produced and a diaphragm plate is to be positioned on this later. A single PZT layer is secured over the diaphragm plate and is subsequently separated into discreet PZT elements by removing material. Subsequently, the diaphragm plate is positioned over the chamber plate and secured, with a number of further work plates being arranged thereunder.
Every rectangular chamber has a delivery channel and a nozzle as well as an oscillation plate with a piezo-ceramic element allocated to it. A disadvantage, however, is that the pressure waves occurring in the ink delivery and in each chamber can cause cross-talk onto further printing chambers. This cross-talk can be subsequently eliminated only as a result of extremely complicated measures, so that these ink jet printing heads are ultimately composed of many individual plates that must be manufactured in a complicated and expensive manufacturing process.
German 34 45 761 likewise discloses a method for manufacturing a transducer arrangement composed of a single plate of transducer material. After coating the lower surface of the plate with a diaphragm layer, a removal of material from the upper surface of the plate of the transducer material ensues in order to generate separate regions that are arranged on the diaphragm above every printing chamber (area: 25.4 mm.times.2.54 mm). The necessity of producing adhesion between the individual transducer elements and the diaphragm with glue is thus eliminated and the uniformity of all spacings is improved. The resulting nozzle spacing, however, continues to be relatively large in a printing head manufactured in this way.
U.S. Pat. No. 4,703,333 also discloses ink jet printing heads manufactured with face shooter modules arranged obliquely offset above one another, producing for an inclined arrangement relative to the surface of recording medium. Ink jet printing heads having an inclined arrangement relative to the surface of a recording medium produce a more uniform recording even given a fluctuating thickness of the recording medium. The manufacture of such printing heads, however, requires a multitude of manufacturing steps. It is difficult to assure the required precision given such a complicated overall structure of each and every printing head. The electrical drive of such printing heads having nozzle rows offset relative to one another which is required during operation is just as complicated. Due to a required, minimum size of the ink chambers, the minimum spacings between the nozzles cannot be additionally reduced even given a mutually offset arrangement of two rows of chambers having nozzles with a slight nozzle density in each nozzle row.
Twice the nozzle density in one row (compared to the density achieved in face shooter ink jet modules with two groups of ink chambers arranged symmetrically relative to the nozzle line) is achieved in a different way in the solution disclosed in U.S. Pat. No. 4,525,728, for an edge-shooter ink jet printing module having one respective nozzle row per chamber plate. Under certain circumstances, the dimensions of the chambers and channels can be further miniaturized. The longitudinal axes of the relatively long ink chambers thereby lie in the direction of the ink jet, whereas the width of the ink chambers is extremely diminished. A problem which then arises, however, is the manufacturing step of applying the PZT elements. The tolerances to be observed are extremely small.
In order to achieve twice the imaging density, it has been proposed in pending German Application P 42 25 799.9 to arrange a plurality of chambers offset horizontally and vertically relative to one another. In this arrangement, however, the channels leading to the nozzles from the distant, lowest level are longer than the channels from the upper, closer level, leading to a phase shift of the individual ink jets that must be electronically compensated. Moreover, the piezo-crystals must exert greater forces due to the extremely long channels, so that these are more likely to fail than other piezo-crystals. In the face shooter ink jet printing head, the channel lengths are shorter and essentially identical as a result of a symmetrical arrangement of all ink chambers in one plane, so that the disadvantage as set forth above is avoided, but at the expense of the resolution.